The present invention relates generally to means for controlling electric power, and more particularly to apparatus for manually controlling the rate at which electric power is furnished to a load from a source of direct current.
It is frequently desirable to manually control the rate at which electric power is furnished to a load. For example, the speed of an electric motor or the brightness of an electric lamp can be varied by controlling the flow of electric power into the device. Electric power can be controlled by means of a rheostat connected in series with the load to control the flow of electric current therethrough, but rheostats are physically bulky, they waste electricity, and they tend to dissipate large amounts of heat, especially in high power applications. If alternating current ("AC"), such as ordinary household power, is available, then the disadvantages of rheostats can be avoided by employing a solid state device such as a silicon controlled rectifier rather than a rheostat to control the flow of current through the load. However, if AC power is not available, then direct current ("DC") from a battery or the like must be used, and solid state devices have not been readily adaptable to the control of DC power.
It has been proposed ("Engineer's Mini-Notebook: 555 Timer IC Circuits" at page 27, by Forrest M. Mims III, published by Radio Shack) to vary the apparent brightness of a DC-powered lamp by applying a variable-frequency square wave to the lamp. However, as the frequency is reduced a flicker may become visible, and such a flicker may not be acceptable. Moreover, this proposal does not address the problems of controlling DC power for other loads such as motors or other inductive devices. Accordingly there remains a need for a compact and efficient means to control the rate at which DC power is furnished to a load.